First of all- a big hug and 
I was in a similar situation to you and it is the absolute worst feeling in the world when you think your child may have an issue. It is always there at the back of your mind no matter what you are doing and you just want someone (preferably a professional!) to say "your child will be absolutely fine and will talk". Unfortunately no one can say at this stage whether he will talk or not.
My son is 20months and he doesn't talk, point, follow a point, have much interest in his siblings or other babies. In terms of what I feel helped/hopefully will help me:
-Focus on what he CAN do
Positives to focus on for your son:
Clapping and waving are the first signs of pre-verbal communication so that's great!
He is able to understand certain things-receptive understanding come first before expressive communication and as long as he can understand a few words and instructions then that's also great.
I am in a Lewisham borough and they give speech therapy from 18months old. However, I was told by various portage workers, gp, and even pediatrician herself that it's only from 2years old. Call up the local speech and language therapy department and ask to speak to a therapist to find out from them what age they accept referrals.
-private speech therapy
While i was waiting ti get off the Nhs waiting list i got a private therapist. 1st one was recommended and apart from being very positive did not give me any practical advise on how to help my son. 2nd therapist gave me some good tips on communication.
Apparently aba is a controversial therapy but I found an amazing consultant and I do the aba programme at home myself with no tutors so that makes it slightly more cost efficient. Search mumsnet talk pages for more info-as there is loads on here.
- Hanen have some good books on communication.
-the earlier intervention the better. So find different ways, online/mumsnet to help a toddlers communication.
-I'm still in a hard place. Everyday i wish my baby was able to at least say "mama" or even look at me when my dh says "look, there's mummy".
Help your son as much as you can and always keep on focusing on the positives.
Don't downplay what a great thing it is that his understand is improving!